linux-kernel - Re: dmaengine/Query: What about scatter/gather for mem to mem transfers.

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Message-ID: <4EE99A15.8010608@st.com>
Date:	Thu, 15 Dec 2011 12:26:21 +0530
From:	Pratyush Anand <pratyush.anand@...com>
To:	"Koul, Vinod" <vinod.koul@...el.com>
Cc:	"Williams, Dan J" <dan.j.williams@...el.com>,
	Viresh KUMAR <viresh.kumar@...com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Shiraz HASHIM <shiraz.hashim@...com>,
	Armando VISCONTI <armando.visconti@...com>,
	Deepak SIKRI <deepak.sikri@...com>,
	Vipin KUMAR <vipin.kumar@...com>,
	Vipul Kumar SAMAR <vipulkumar.samar@...com>,
	Vincenzo FRASCINO <Vincenzo.FRASCINO@...com>,
	Mirko GARDI <mirko.gardi@...com>,
	Rajeev KUMAR <rajeev-dlh.kumar@...com>,
	Amit VIRDI <Amit.VIRDI@...com>,
	Bhupesh SHARMA <bhupesh.sharma@...com>,
	"linus.walleij@...aro.org" <linus.walleij@...aro.org>
Subject: Re: dmaengine/Query: What about scatter/gather for mem to mem transfers.

On 12/15/2011 10:54 AM, Pratyush Anand wrote:
> On 12/15/2011 10:36 AM, Koul, Vinod wrote:
>>>
>>> Hello Vinod/Dan,
>>>
>>> Please write your opinion.
>> I am on vacation, but will reply briefly here, more next week
>
> Thanks for your quick reply.
>
>>>
>>> Regards
>>> Pratyush
>>>
>>> On 12/14/2011 12:17 PM, Pratyush Anand wrote:
>>>> Hello Vinod/Dan,
>>>>
>>>> In continuation to the scatter/gather requirement:
>>>> We might need some generic transfer where source and destination
>>>> address may be overlapped and also gap between two chunk of source and
>>>> destination might not be same.
>>>>
>>>> For examaple,
>>>> Transfer size is - -0x4000
>>>> Our Src is something like this:
>>>> 0x1000 -- 0x2000
>>>> 0x3000 -- 0x5000
>>>> 0x6000 -- 0x7000
>>>>
>>>> and dst is something like this:
>>>> 0x6000 -- 0x8000
>>>> 0x9000 -- 0xB000
>> So why can't it be split like:
>> 0x1000--0x2000 => 0x6000 --0x7000
>> 0x3000 -- 0x4000 => 0x7000 - 0x8000
>> 0x4000 -- 0x5000 => 0x9000 - 0xA000
>> 0x6000 -- 0x7000 => 0xA000 - 0xB000
>>
>> That way existing mechanism would work well for you.
>> You need to split the chunks properly, which is what dma would do anyway
>>
>
> Yes, they can be split like this, but then splitting onus will go on dma
> user driver, and so there would be replication of similar logic at
> several places. Therefore, I was thinking to make device_prep_dma_sg as
> generic by adding these flags.

I see one more issue in using device_prep_interleaved_dma.

Src and Dst address has been allocated in user space.
Now a kernel module extracts physical addresses from these pages and
prepares a sg list, which it submits to DMA.
These addresses would be virtually contiguous and incrementing. But, I 
am not sure if they are always physically incrementing too. If they are 
not guaranteed to be incrementing, then I see issue.

Otherwise also, a situation can arise when scattered memory is not 
always incrementing or decrementing in the same sg list.

>
> Regards
> Pratyush
>
>> --
>> ~Vinod
>

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